Actually, the Bible DOES say that Mary was sinless. In Luke 1:28, when the angel appeared to Mary, he addressed her as “full of grace” which is a loose translation into English which misses much. The actual Greek word used there was
kecharitomene which means that Mary was “full of grace” from the first moment of her existence (conception), permanently through the rest of her life. This is where we get the doctrine of the Immaculate Conception. Here’s an article on it:
catholic.com/library/Immaculate_Conception_and_Assum.asp
You might further explain that no one before the age of reason can sin, since sin is knowingly saying, “No!” to God. Therefore, all who are not yet of the age of reason are sinless. If they die before they reach the age of reason, they will have lived their whole lives sinless. Same for the mentally handicapped.
Heb. 4:15 says that Jesus was sinless.
And, from a logical standpoint, since Jesus is God, He could not have sinned, since He could not have said, “No!” to Himself.